The invention relates to a high-frequency amplifier comprising at least one differential stage which comprises a first transistor and a second transistor. The emitters, are coupled to each other, and to a first current source, whose bases receive the input signals. The collectors are connected to the emitters of a third transistor and a fourth transistor respectively, whose collectors are each connected to a supply-voltage source by a first bias resistor and a second bias resistor respectively. The bases of the third and the fourth transistor are connected to the collectors of the fourth and the third transistor respectively.
An amplifier of the type defined in the opening paragraph is known from German Patent Application No. DE 3,121,314 (PHILIPS GLOEILAMPENFABRIEKEN) published on Apr. 22, 1982. The patent application describes a high-frequency amplifier which is specifically suitable for powering a frequency divider and in which the bases of the third and the fourth transistor are connected to the collectors of the fourth and the third transistor respectively via base resistors. The base resistor's value as well as the value of the collector resistors and that of the bias current are selected in conformity with formulas given on page 6 lines 5 to 31 so as to optimise the high-frequency behaviour and at the same time also eliminate the d.c. component in the ideal case (page 6, lines 15 to 19).
Multi-stage high-frequency amplifiers are known which are in particular suitable for use as input amplifiers for a frequency divider and which have a high gain of the order of, for example 20 to 30 in the central part of their operating-frequency range. However, these amplifiers have a pass band which is limited towards the higher frequencies where the response curve exhibits a comparatively strong roll-off (-18 to 24 dB/octave). Indeed, with such amplifiers it is difficult to obtain a gain higher than unity at the maximum operating frequency of the amplifier if this frequency, which is for example of the GHz order, substantially approaches the intrinsic cut-off frequency of the transistors being used, for example when it reaches a fifth or a third of said cut-off frequency. At these frequencies the transistors produce very annoying additional attenuations. By means of a single-stage amplifier it is possible to obtain a broad pass band, but the gain in the central part of the pass band is then limited to a value of the order of, for example, 4 to 5.
On the other hand, if two stages of the same type are cascaded the amplifier thus obtained will have a gain smaller than unity at a frequency lower than the frequency for which one stage alone has a gain equal to unity. This is because the input impedance of the stage forming the output stage of the amplifier gives rise to an additional attenuation of the input stage, which attenuation adds to said attenuation caused by the transistors. In other words, because it is coupled to the output stage, the input stage has a pass band which is limited towards the high frequencies, and any coupling is found to be problematic.